The present invention relates to fluid transfer systems, and more particularly to the transfer of liquids by pipette such as in chemical analyzers and mixing apparatus.
In a conventional pipette liquid transfer system, the pipette is lowered into contact with a sample liquid in a first container, a measured volume of the liquid ia drawn into the pipette, the pipette is raised and positioned over a second container, and the liquid is dispensed into the second container. The chemical analyzers typically process clinical samples from patients whose medical condition is being diagnosed. Thus a limited quantity of each sample is available and the analysis must be highly accurate and reliable. In order to transfer a small, precisely measured volume or aliquot of the liquid from a small sample, it is important that the pipette be very small in size and lowered only far enough to maintain contact with the liquid.
Such conventional systems exhibit various disadvantages, such as the following. In some apparatus, a pair of electrical probes is lowered with the pirpette into the container for sensing the liquid level. The pipette and multiple probes, each contacting the liquid, can pick up large, random, unmeasured quantities of the liquid, causing measuremenr errors. The combination of the pipette and the probes is bulky, requiring a large container that exposes a large surface area of the liquid to contamination; moreover, the combination is expensive to build and hard to align.
In other conventional apparatus, a probe is excited in a capacitive bridge circuit, a probe voltage being attenuated wnen the probe contacts a liquid that is capacitively coupled to ground. Other sensitive devices in the analyzer and nearby instruments are thus subject to electrical interference. These circuits are also sensitive to stray capacitances associated with the apparatus and external sources such as the hand of an operator of the apparatus. Further, they have a cumbersome alignment procedure that requires balancing of the bridge circuit. Thus the skill of an operator is a major factor in the speed and accuracy of the apparatus.
In an effort to solve some of these problems, further apparatus has the probe grounded for loading an oscillator when the probe contacts a liquid that is capacitatively coupled to the oscillator. The sensivity is reduced because the oscillator output changes only relatively slightly when the liquid contacts the probe. Consequently, elaborate circuits are used in this apparatus.
Thus there is a need for a level sensor for liquid transfer apparatus that reliably and precisely senses contact between a pipette and a sample liquid, and is simple and inexpensive to manufacture.